This invention relates to a beam quality detector system and more particularly to a system utilizing a single image detector.
Image motion detectors capable of tracking the trajectory of a plane or other moving objects, utilizing radiation emanating therefrom, are known in the art. Typically, radiation emanating from a moving body is continuously focused onto a rotating chopper or scanning disk which includes alternate zones of different transparency to the incident rays which modulate the intensity of the rays passing through the rotating disk according to the coordinate position of the alternate zones in the image field. The modulated intensity is detected by optical means which generate amplitude and frequency information proportional to the coordinates of the image point on the disk.
Schmutz in U.S. Pat. No. 3,307,038 discloses a rotating chopper disk adapted for periodically interrupting the passage of radiation from a search object to a detector. The rotating disk has an outer circular ringshaped track having the form of a radial gap pattern which is periodic in the circular direction of the track movement and which repeats itself in sequential sector-forming sections. A dividing line extending diagonally across the sector-forming sections divides the sector-forming section into two fields wherein the radial gap pattern in each field is different. Motion of the image across the dividing line results in a frequency variation in an output signal from the detector which is analyzed by means well known in the art to determine the direction of the motion of the search object.
Astheimer in U.S. Pat. No. 3,090,869 discloses a motion detector device which utilizes a reticle in the form of a drum or a disk. The reticle includes bands of opaque bars and clear bars interspersed with one another wherein the bands are disposed at an angle of forty-five degrees to the reticle travel direction and at right angles to one another. On the outer edge of the reticle are provided two sets of phase reference patterns for elevation phase and azimuth phase references. The error signal is developed with respect to the two orthogonally disposed bands which produce frequency and phase variations porportional to the radial and azimuthal motion of the image of the object being tracked.
Malueg in U.S. Pat. No. 3,950,099 discloses a two-axis image motion detector adapted for detecting two separate velocity channels through a single optical channel with a single photodetector. A single disk rotates at a substantially constant rotational speed and contains a pair of orthogonal grid patterns of different spatial frequencies. In the preferred embodiment each grid pattern is disposed at a forty-five degree angle with respect to a radial axis extending from the center of the disk through the grid pattern. An image focused on the grid pattern modulates the carrier frequencies determined by the grid patterns thereby generating a composite output in a single photodetector representing the sum of the two image motion channels. Standard filtering and demodulating techniques are utilized to generate a DC voltage proportional to the image rate for each of the orthogonal axis.
Prior art devices are adapted for sensing the radial and azimuthal motions of an object but are not adapted for determining the optical quality of a beam of radiation. In particular, beam astigmatism and defocus measurements are not possible and the prior art devices are not adapted for providing error signals to a feedback circuit capable of minimizing the degradation of the optical quality of a beam due to beam jitter, astigmatism or defocus.
Cornwell in U.S. patent application Ser. No. 973,338 filed on even date herewith and held by a common assignee discloses an apparatus for monitoring the optical quality of a beam wherein the apparatus includes a disk, adapted for rotation about a central axis, having a plurality of slit sets symmetrically disposed circumferentially within the disk at a radial position wherein each slit set includes a first slit and a second slit disposed in an orthogonal relationship to one another and symmetrically disposed about a radial axis extending from the plurality of slit sets to the central axis and a third slit disposed between the first slit and the second slit bisecting the angle therebetween and superimposed on he radius line. A synchronization source disposed proximate the front surface of the disk is adapted for providing a synchronization signal through the same slit through which the radiation is focused to provide timing signals for data reduction. Beam jitter is obtained from data provided by the first and second slits within a slit set, beam astigmatism is provided from data obtained from the first, second and third slits of a slit set and beam defocus information is obtained from data provided by the variable position of the minimum slit width portion of each slit set within a triplet group of slit sets.
Unfortunately a scanning disk with a plurality of slit sets circumferentially disposed about the scanning disk at a single radical position may suffer a compromise in performance if more than one optical measurement is required. This occurs when the optimum operating conditions for the measurement of a single parameter of the optical quality of a beam of radiation may require a different slit configuration and/or dimensions than the requirements for the measurement of a second parameter. This results in slit sets having a compromise configuration and/or dimensions suitable for use in measuring all of the parameters defining a combination of measurements of the optical quality which cannot yield optimum values for each parameter which may detract significantly from the achievable overall performance of the apparatus. Additionally the alternate measurement of the optical quality of beams having high and low signal-to-noise ratios would typically require changing the rotation rate of the disk. This requires a prohibitively long change time for many applications due to the inertia of the spinning disk to change in rotation rate.